The present invention relates to a nitride semiconductor and a method for manufacturing the same, and in particular, to a nitride semiconductor device that is formed by the manufacturing method and expected to be applied to a field of optical information processing.
A nitride semiconductor containing nitrogen (N) as an element of the group V is considered to be hopeful as materials for short wavelength luminous element because its band gap is relatively large. Among the nitride semiconductors, a gallium nitride-based compound semiconductor that is represented by general formula AlxGayInzN (wherein 0≦x, y, z≦1, x+y+z=1) has been studied hard. Blue light emitting diode (LED) device and green light emitting diode device have already been in practical use.
A short wavelength semiconductor laser device with about 400 nm of oscillation wavelength has been desired eagerly in order to extend a capacity of optical disk unit. A nitride semiconductor laser device using a gallium nitride-based compound semiconductor has attracted attention and has been reaching the level of practical use.
A nitride semiconductor laser device is formed by crystal growth on a substrate made of gallium nitride (GaN). Nevertheless, it is difficult to produce a substrate (wafer) made of silicon (Si) or gallium arsenide (GaAs). For this reason, for example, as reported in IEICE TRANS. ELECTRON, VOL. E83-C, No. 4, PP.529-535(2000), on a substrate made of sapphire, a semiconductor laser made of gallium arsenide (GaN) is grown to a thickness of 100 μm or larger by eptaxial lateral overgrowth technique utilizing hydride vapor phase epitaxy (HVPE) or metal organic vapor phase epitaxy (MOVPE). Then, the sapphire substrate is removed and thus a substrate made of gallium nitride is produced.
FIG. 12 shows a cross-sectional structure of a conventional gallium nitride-based semiconductor laser device that laser oscillation has been accomplished.
As shown in FIG. 12, on a substrate 101 made of GaN, an n-type contact layer 102 made of n-type Al0.015Ga0.985N which lattice-matches the substrate 101, a crack suppression layer 103 which has a thickness of about 0.1 μm and made of n-type Ga0.95In0.05N, an n-type superlattice cladding layer 104 made of n-type Al0.15Ga0.85N/GaN, an n-type optical guide layer 105 made of n-type GaN, a multi-quantum well (MQW) active layer 106 made of GaInN, a current blocking layer 107 made of p-type Al0.2Ga0.8N, a p-type optical guide layer 108 made of p-type GaN, a p-type superlattice cladding layer 109 made of p-type Al0.15Ga0.85N/GaN and a p-type contact layer 110 made of p-type GaN are successively formed by growth.
The semiconductor laser device relating to a conventional example is characterized by having the crack suppression layer 103 made of GaInN between the n-type contact layer 102 and the n-type superlattice cladding layer 104.
Because of the crack suppression layer 103, lattice distortion that occurs between the n-type superlattice cladding layer 104 at which cracks easily occur because of its smallest lattice constant among a plurality of gallium nitride-based semiconductor layers including the active layer 106 for structuring a laser structure and its largest film thickness, and the n-type contact layer 102 which lattice-matches the substrate 101 is reduced, and generation of cracks caused by the lattice distortion occurred at a time of forming the laser structure is suppressed.
Although the aforementioned conventional gallium nitride-based semiconductor laser device is provided with the crack suppression layer 103 for alleviating the lattice distortion of the n-type contact layer 102 and the n-type cladding layer 104 between them, the close relationship between lattice constants of them is not considered. For this reason, adjusting a composition of In in the crack suppression layer 103 or its film thickness, or optimizing growth conditions is the only method for suppressing cracks generated at the n-type superlattice cladding layer 104.
For example, crystal defect or dislocation easily occurs at the crack suppression layer 103 because it generally grows at a temperature that is lower, by 150 to 300° C., than a growth temperature of semiconductor layer such as the n-type cladding layer 104 or the like and a temperature of the crack suppression layer 103 is increased when the n-type cladding layer 104 and the n-type optical guide layer 105 are grown on the crack suppression layer 103 and thus the thermally instable crack suppression layer 103 is exposed to a higher temperature than its growth temperature. Accordingly, crystallization property of the crack suppression layer 103 is inferior and its reproducibility at a time of its growth is poor. Consequently, it is difficult to produce the crack suppression layer 103 itself As a result, the n-type superlattice cladding layer 104 growing on the crack suppression layer 103 receives defectives generated at the crack suppression layer 103 and thus it is not easy to form a laser structure with high quality semiconductor crystalline layers.
When the crack suppression layer 103 is provided between the n-type contact layer 102 and the n-type superlattice cladding layer 104, the crack suppression layer 103 with inferior crystallization property serves as a current path. Thus, there arises a problem in that a reliability of element at a time of high output operation is decreased, for example, a reverse withstand voltage is reduced.
Further, an effect of suppressing cracks generated at the n-type contact layer 102 which lattice-matches the substrate 101 made of gallium nitride cannot be expected for the conventional crack suppression layer 103.
Moreover, when such device is applied to a semiconductor laser device, in particular, a laser device for optical disk that is capable of reading and writing, there arises a problem in that spontaneous emission light leaking from the active layer 106 during low output operation at a time of reading becomes a source of noise. The aforementioned conventional gallium nitride-based semiconductor laser device does not consider suppression of such spontaneous emission light.